The present invention relates to an improved cyclone dipleg check valve which is capable of improving the operation of the dipleg under a variety of operating conditions.
In a cyclone separator, a suspension comprising a gasiform material with entrained finely divided solid material is introduced into the separator in a tangential manner that imparts a spiral or centrifugal swirling motion or action to the suspension. The centrifugal action causes the solids to be thrown to the outer wall of the cyclone separator and they then exit the separator via a standpipe or dipleg means. These cyclones are particularly useful in connection with organic reactions employing fluidizable catalytic particles, such as the catalytic treatment of petroleum fractions by cracking, the synthesis of hydrocarbons from carbon monoxide and hydrogen, the conversion of methanol to liquid hydrocarbons or other fluid catalyst operations known in the art.
In these operations, such as, for example, riser catalytic cracking operations, enormous amounts of solid catalyst particles suspended in gasiform materials are handled in riser catalytic cracking units and it is necessary to rapidly separate the suspensions into a catalyst phase and a gasiform phase after the suspension from the conversion operation has traversed the riser unit or the conversion zone. The dipleg or standpipe means of the cyclone separator is usually equipped with a valve means which allows the accumulated solid catalyst particles to be discharged into the space below the dipleg once the weight of the accumulated particles in the dipleg reaches a predetermined limit. The valve means may sometimes be equipped with a counterweight to regulate the weight of the solid particles necessary to open the valve means.
The most common types of valve used for that purpose are trickle valves, flapper valves and flapper valves with a counterweight, the weight of which may be adjusted for a particular application. These are well known in the art as shown and discussed in, e.g., Zenz and Othmer, "Fluidization in Fluid Particle Systems," Rheinhold Publishing Corp., 1960, pages 410-415.
In an operation involving heavily loaded cyclones, such as riser cyclones or primary cyclones, the flapper valve may stay open permanently at normal operating conditions; however, the degree of opening of the valve must be restricted by the counterweight, so that an inventory of catalyst is retained inside the dipleg.
With lightly loaded cyclones, such as secondary cyclones, the flapper valve needs to open and close intermittently in order to prevent the aforementioned undesirable flow of gaseous reactants in the dipleg.
This flow of gas, in the case of riser cyclones, is downwards and can affect reaction selectivity and reactor vessel entrainment. In the case of primary or secondary cyclones, the gaseous flow in the dipleg is upwards; it can seriously interfere with the operational efficiency of the separator and may even prevent its satisfactory operation.
It is also known in the art that under certain operating conditions, the lower end of the flapper valve may be submerged into the catalyst bed which is present within the reaction vessel. In such a "submerged" operation, it is preferred that the flapper valve remain opened because even partial closure of the valve would restrict the flow of solids from the dipleg. However, the freedom of motion of the flapper valve is restricted by the surrounding catalyst fluid bed, and so there is always a potential for failure to open, or to open sufficiently. This will cause catalyst to fill the dipleg completely, and to back up into the cyclone, leading to excessive catalyst carryover.
Furthermore, during start-up of the submerged operations, the catalyst bed is usually too low to seal the diplegs for a long period of time, due to a low catalyst inventory. Consequently, a large amount of catalyst is often lost through unsatisfactory separator operation. Moreover, this lost catalyst has been known to cause serious problems, such as air pollution and fouling of downstream product recovery equipment.
Trickle valves may be employed under such circumstances: however, they leak and so they do not seal effectively. Furthermore, the freedom of motion of a trickle valve may be restricted by the surrounding catalyst fluid bed, leading to complete filling of the dipleg and excessive catalyst carryover.
Thus, the need still exists for a flapper valve which can effectively work under a variety of operating conditions, including both submerged and non-submerged operation.
Accordingly, it is an object of the present invention to provide such a flapper valve.